1. Field of the Invention
The present claimed invention relates to the field of semiconductor wafer processes. More specifically, the present claimed invention relates to Isopropyl Alcohol (IPA) tanks used to clean semiconductor wafers.
2. Prior Art
During semiconductor wafer fabrication processes, the semiconductor wafers require periodic cleaning. One common method of cleaning uses an IPA tank. The IPA tank contains very pure liquid IPA in the bottom thereof. Cooling or condensation coils are located around the inner edge of the IPA tank above the level of the liquid IPA contained within the IPA tank. The liquid IPA in the bottom of the IPA tank is heated so that a vapor of IPA is produced. The vaporized IPA rises within the IPA tank until it reaches the level of the cooling coils. When the vaporized IPA reaches the level of the cooling coils it condenses back into liquid IPA, referred to as "condensate", and falls back into the reservoir of liquid IPA located in the bottom of the IPA tank.
During a typical cleaning operation, a group or "rack" of semiconductor wafers to be cleaned are lowered into the IPA tank. The rack of wafers are located at such a level within the tank such that the cooling coils peripherally surround the rack of wafers. Thus, the vaporized IPA condenses on the semiconductor wafers. The IPA removes contaminate particles and facilitates efficient removal of residual deionized water which may remain on the semiconductor wafers as a result of previous process steps. The contaminated condensate drips from the semiconductor wafers back towards the reservoir of very pure IPA. Commonly, a flat bottom catch tray is arranged within the IPA tank between the reservoir of very pure IPA and the semiconductor wafers. In so doing, drops of contaminated condensate which drip from the semiconductor wafers are caught by the flat bottom catch tray and do not contaminate the reservoir of very pure IPA in the bottom of the catch tank. The drops of contaminated IPA are removed via a drain in the flat bottom catch tray. Thus, the catch tray functions as a liquid/vapor IPA tank separator.
Unfortunately, conventional IPA tanks employing flat bottom catch trays have severe drawbacks. In instances where the flat bottom catch tray is in contact with the reservoir of very pure IPA, the top surface area of the reservoir of IPA, from which evaporation would take place, is dramatically reduced. Furthermore, by placing a flat bottom catch tray above the surface of the very pure IPA reservoir, the flow of vaporized IPA is significantly restricted. That is, upward flow of vaporized IPA occurs only from those portions of the IPA reservoir which are not covered by the flat bottom catch tray. Thus, upward flow of vaporized IPA occurs only from those portions of the IPA reservoir located between the outer edge of the flat bottom catch tray and the side of the IPA tank.
The upward flow of vaporized IPA is essential for effective cleaning of the semiconductor wafers. If the flow of vaporized IPA is not great enough, premature drying of the deionized water may occur resulting in the deposition of deleterious residue on the semiconductor wafers, or inadequate particle removal. Additionally, when the rack of semiconductor wafers, is placed into the IPA tank, vaporized IPA present within the tank may be displaced and forced out of the IPA tank. Thus, additional vaporized IPA must be produced to replace the displaced vaporized IPA. The time that is required to replace the lost vaporized IPA is referred to as the recovery time of the IPA tank. If the upward flow of the vaporized IPA is obstructed by a flat bottom catch tray, the recovery time of the tank is adversely affected thereby increasing the possibility of inadequate particle removal and/or premature drying of any deionized water.
In an attempt to deal with the problems associated with restricted flow of vaporized IPA, alternate types of catch trays or liquid/vapor IPA tank separators have been employed. Typically, these trays are formed of sloped plates placed above the top surface of the IPA reservoir. Although newer trays do not contact as much of the top surface area of the reservoir of IPA, these trays still greatly restrict the flow of vaporized IPA at the center of the IPA tank. As a result the flow of vaporized IPA is again forced to the outer edges of the IPA tank.
Thus, the need has arisen for a liquid/vapor IPA tank separator or "catch tray" which does not contact a large portion of the top surface area of the reservoir of IPA, which does not significantly restrict the upward flow of vaporized IPA, and which does not force the flow of vaporized IPA to the edges of the IPA tank.